1. Field of the Invention
The present invention relates to a switching-element drive circuit.
2. Description of the Related Art
When designing a power converter, the magnitude of noise occurring from the power converter, frequency components thereof, and the like need to be taken into account. Considering a power converter used in an air conditioner as an example, noises occurring from this type of power converter vary under influences of installation conditions, power supply conditions, operation statuses, individual variation of power converters, and the like. Noise (a noise current) occurring in the power converter is radiated from the power converter as radiation noise, and frequency components of the radiation noise include (specific) noise components equal to integral multiples of a fixed high frequency. Because the frequency components of the radiation noise are the specific frequency components, the radiation noise has a high peak value. Furthermore, a radiation path of the radiation noise is quite complicated. For these reasons, protection (design) against noise is quite difficult in the power converter.
Meanwhile, when a power converter is manufactured and shipped, a reference level with a sufficient margin towards a standard reference needs to be set. Accordingly, it is necessary to provide sufficient measures for suppressing an amount of occurring noise, which leads to increase in sizes of noise suppression parts and increase in the number of the parts.
As a measure for suppressing the amount of occurring noise, reduction in the switching speed of a switching element is conceivable. When the switching speed of a switching element is reduced, the amount of occurring noise can be decreased; on the other hand however, a different problem that a switching loss is increased occurs.
In a switching-element drive circuit described in Japanese Patent Application Laid-open No. 2010-252451, a technique of a switching-element drive circuit of a power converter is disclosed that includes a switching-element drive unit driving a switching element according to a specified current value and controlling a main current flowing through the switching element, and that controls a time rate of change in a drive voltage to be applied to the switching element according to the specified current value.
However, because the time rate of change in the drive voltage to be applied to the switching element is changed according to the specified current value in Japanese Patent Application Laid-open No. 2010-252451, the switching speed cannot be changed within one cycle of an AC power supply. Accordingly, in the conventional technique described in Japanese Patent Application Laid-open No. 2010-252451, it is difficult to disperse noise frequency components under certain load conditions. Furthermore, while a certain effect of reduction in the noise amount is achieved in this conventional technique, the peak component of the noise still remains. Accordingly, this conventional technique is not effective as measures against noise to keep the frequency component of the noise peak within the standard reference and a lot of measures against noise are still required.
The present invention has been made to solve the above problems and an object of the present invention is to provide a switching-element drive circuit that can effectively provide measures against noise to keep the frequency component of a noise peak within the standard reference.